The present invention relates to a nonlinear resistor for use in an overvoltage protection device and a method of manufacturing the same. More specifically, the present invention relates to a nonlinear resistor having an electrode and a side-surface high resistance layer and the method of manufacturing the same.
Generally in an electric power system, an overvoltage protection device such as a lightning arrester or a surge absorber is used in order to protect the electric power system by removing overvoltage which is superposed on a normal voltage. In the overvoltage protection device, a nonlinear resistor is mainly used. The nonlinear resistor used herein is characterized in that it exhibits substantially insulating characteristics under a normal voltage and a relatively low resistance when an overvoltage is applied.
The nonlinear resistor of this type has a sintered body. The sintered body is formed of zinc oxide as a main component and at least one type of metal oxide as the additive. The additive is used in order to obtain the nonlinear resistor characteristics. The materials are mixed, granulated, molded, and sintered to form the sintered body. At the side surface of the sintered body, a side-surface high resistance layer is formed in order to prevent a flashover from the side surface when a surge is absorbed. Furthermore, an electrode is formed on each of upper and lower surfaces of the sintered body such that a current flows uniformly through the sintered body.
In the electrode of the nonlinear resistor mentioned above, a ring-form electrode nonformation portion is provided, in most cases, in a circumference portion of the nonlinear resistor in such a manner that an electrode end portion does not overlap with the sintered-body end portion in order to avoid a flashover as much as possible when a large current is supplied.
Methods for forming the electrode nonformation portion are disclosed, for example, in Jpn. Pat. Appln. KOKOKU publication No. 5-74921 and Jpn. Pat. Appln. KOKAI publication No. 8-195303. In these methods, the ring-form electrode nonformation portion is formed in the circumference portion of the nonlinear resistor by applying a rubber mask to the nonlinear resistor when the electrode is formed. Furthermore, in the method disclosed in Jpn. Pat. Appln. KOKAI publication No. 11-186006, the ring-form electrode nonformation portion is formed in the circumference portion of the nonlinear resistor such that the sintered body end portion and the electrode end portion are placed at a distance of 0.01 to 1.0 mm.
Also, in the disclosure of other numerous patent publications and various technical documents, a ring-form electrode nonformation portion is provided in the circumference portion of the nonlinear resistor. As described above, the technique that the ring-form electrode nonformation portion is provided in the circumference portion of the nonlinear resistor is widely known and has been generally employed hitherto.
With the recent remarkable development of information technology in society, the demand for an electric power has increased. In the circumstances, the electric power is demanded to be stably supplied at a low cost. In addition, there is a strong demand for miniaturizing transmission/substation appliances due to the shortage of the space for placing the transmission/substation appliances in urban areas. With the demand for stable supply of electricity to electric power systems and the demand for miniaturization, the requirement for miniaturization of the highly reliable overvoltage protection device has been increased.
To satisfy the demands, the miniaturization of the nonlinear resistor of the overvoltage protection device, has been accelerated in such a manner that the height is reduced as much as possible by increasing a voltage per unit thickness of the nonlinear resistor and the overall size is reduced by improving the energy absorbing ability. As a matter of course, even if the overvoltage protection device is miniaturized, the operation must be stably performed for a long time.
However, as is described in the conventional nonlinear resistor mentioned above, in the case where the ring-form electrode nonformation portion is provided in the circumference portion of the nonlinear resistor in such a manner that the electrode end portion is not overlapped with the sintered body end portion in order to avoid a flashover generated at the time a large current is supplied, a thermal stress generates due to the presence of the electrode nonformation portion, with the result that the sintered body may possibly be broken.
In the nonlinear resistor having an electrode formed at the upper and lower surfaces of the sintered body by forming the ring-form electrode nonformation portion in the circumference, a current flows through the electrode-formation portion when the current is supplied, whereas no current flows through the ring-form electrode nonformation portion around the periphery of the none-linear resistor. It follows that only the temperature of the electrode formation portion increases. Due to the difference in temperature between the electrode formation portion and the electrode nonformation portion, the thermal stress is produced which cracks and breaks the sintered body. As a result, there is a possibility of reducing an overvoltage protection performance of the nonlinear resistor.
Therefore, in the conventional method in which the ring-form electrode nonformation portion is formed in the circumference of the nonlinear resistor, it has been difficult to ensure sufficient protection performance against a surge such as a switching surge, lightening impulse, and overvoltage, although the sufficient protection performance is required when the non linear resistor is miniaturized by increasing the voltage per unit thickness or by reducing the diameter thereof.
To overcome such a problem, it is conceivable that the area of the electrode formation portion is enlarged as much as possible.
However, in the conventional nonlinear resistor, if the electrode is formed so as to extend to or near the side-surface high resistance layer, a flashover is generated at an interface between the sintered body and the side-surface high resistance layer. The flashover is caused by poor adhesive strength of the side-surface high resistance layer to the sintered body at the time an overvoltage surge is applied. Alternatively, the flashover is caused by poor electric insulation characteristics or poor heat resistance of the side-surface resistance layer. Moreover, the ability of the loaded lifecycle may possibly deteriorate due to overvoltage under normal operation conditions in which a voltage is constantly applied.
Therefore, the problems residing in the conventional nonlinear resistor are that a nonlinear resistor having high overvoltage protection performance and a stable ability of a loaded lifecycle cannot be attained.